Research On The Charge Characteristics Of Oil-paper Insulation Of UHV Converter Transformer Under DC And Superimposed Harmonic Electric Fields

The converter transformer is the core part of the UHV DC power grid.It forms the commutation part of the DC transmission system together with the converter valve.Its operation status directly determines the safety of the UHV DC transmission system.Since the main working device of the converter valve is a non-linear element,its electric field environment is more complicated.Unlike ordinary power transformers,its electric field type,harmonic content and corresponding charge behavior are not yet clear.The current research on the charge of UHV converter transformer mainly focuses on DC electric field,polarity reversal electric field and corresponding material modification The research on harmonics is also mainly focused on suppressing the current harmonics,and the research on the harmonics of its own voltage is not comprehensive and specific enough.This article focuses on the charge behavior of converter transformer under DC and harmonic electric fields.The simulation software is used to build the calculation model of UHV DC transmission system,and the voltage waveform of valve side bushing of converter transformer is deeply analyzed.The frequency domain analysis method is used to comprehensively analyze the voltage waveform,harmonic type and content of valve side bushing of converter transformer at different positions.Based on the electro acoustic pulse charge measurement platform,the charge characteristics under the environment of DC and DC superimposed harmonic are studied.The simulation model of ±800kV UHV DC transmission system is built,and the measurement points are set between the transformer and the bushing at valve side.The voltage waveforms of 16 bushing at positive and negative polarity rectifier side,inverter side and valve side of converter transformer are obtained respectively.It is found that the voltage waveforms at different positions are quite different.The calculated voltage waveform is compared with the measured voltage waveform at the corresponding position,and they are basically consistent.For the position of the bushing on the valve side of the high-pressure valve hall,it is greatly affected by the DC voltage,and less affected by the AC and harmonic electric fields.The AC and DC ratios are 1:7 and 1:5 respectively.For the low-voltage valve hall,the ratio of AC and DC voltage at the star-star connection bushing position is 1:3,and the ratio of AC and DC voltage at the star-corner transformer bushing position is about 1:1.For its harmonic content,the low-end star-corner connection of the rectifier side and the valve side winding of the converter transformer are mainly affected by the harmonics of 150Hz,250Hz,300Hz,350Hz,550Hz,600Hz.On the inverter side,there is a certain difference between positive and negative harmonics.Among them,the low-end star-corner connection converter transformer valve-side winding of the positive polarity inverter side is 150Hz,250Hz,350Hz,600Hz,650Hz,750Hz,which is less affected by the 550Hz harmonics.On the inverter side,there is a certain difference between positive and negative harmonics.The windings on the valve side of the low-end star-corner converter transformer on the negative inverter side are mainly affected by the 150Hz,250Hz,300Hz,350Hz,550Hz,600Hz,650Hz harmonics,and the 750Hz harmonic content is relatively low.Carry out research on charge characteristics in DC and AC-DC electric fields.Firstly,the charge characteristics under different electric field intensity and polarization time were measured.It was found that at a lower electric field intensity,the charge injection effect and polarization effect were small,and with the increase of time,a small amount of heteropolar charges were accumulated inside.However,in the case of higher electric field intensity,its polarization and injection are both improved,and the polarization of heteropolar charge is faster,and the accumulation of heteropolar charge is presented in the early stage.In the later stage of pressure,due to the limited ionizable charge in the cardboard and the limited polarization of charge,a large number of charges of the same polarity will be injected in the later stage.Because there are many charge traps in oil-paper insulation,the internal charge accumulation is more obvious For the charge behavior of oil-paper insulation under AC/DC electric field,the relationship between surface charge and field strength is great.At the same polarization time,the surface charge increases obviously with the increase of field strength,while the change of internal charge accumulation is not obvious.